The conversion of analog to digital signals and digital to analog signals is known. Conversion of analog to digital signals may be done utilizing Sigma Delta Modulators, successive approximations and/or flash circuits. The conversion of digital signals to analog signals may be done using complimentary circuitry to Sigma Delta Modulators, successive approximation circuits, and/or flash circuits.
A Sigma Delta analog to digital converter (.SIGMA..DELTA.ADC) includes a first or second order Sigma Delta Modulator and a quantizer. As is known, a first order Sigma Delta Modulator includes a single modulation stage while a second order Sigma Delta Modulator includes two modulation stages. To regulate the Sigma Delta Modulator, the output of the quantizer is fed back to each stage of the said Sigma Delta Modulator via digital to analog converter modules. While .SIGMA..DELTA.ADC provide a relatively high-speed analog to digital conversion there are some limitations for high fidelity applications. For instance, when the analog signal is a DC value for a relatively long time (much greater than the sampling rate), a noise component having a frequency of one-half the switching frequency is present. The noise can be injected into a voltage reference signal applied to the digital to analog converter feedback module and propagates through the ADC due to imperfections in the parts comprising the ADC. For example, amplifiers within the Sigma Delta ADC include offset that allow the noise to pass.
In many Sigma Delta Modulator ADC, this noise is of minimal consequence since the analog input is rarely, if ever, a DC value for a significant period of time. In digital audio applications, a DC analog signal may be present for a sufficient period of time causing the noise signal to be produced. Typically, the noise produced will be in the audio range thereby being rendered audible by the audio equipment.
Therefore, a need exists for a method and apparatus that substantially reduces one-half sampling frequency noise that is generated when the analog input to a Sigma Delta ADC is a DC value.